Direct current to direct current (DC to DC) converters which employ a half-bridge configuration and which operate in a self-oscillating mode, are commonly realized by converter topologies which are referred to in the art as resonant pole converters. These converters have zero voltage switching and are classified into three categories: linear resonant pole, non-linear resonant pole, and double non-linear resonant pole.
Resonant pole converters are characterized by simple circuit designs which integrate some of the circuit parasitics into the required functionality. However, operation of such converters at higher frequencies becomes extremely difficult to achieve without sacrificing efficiency and without producing high levels of electromagnetic interference. The frequency of operation for prior art converters is limited to approximately 200 kHz. In order to combat the aforesaid limitations, the double non-linear resonant pole converter has been developed which has lowered switching losses, reduced parasitic oscillations, etc., as disclosed in the "Study of a Generic Topology for a Double Non-Linear Resonant Pole Converter" by J. R. DeVilliers. J. D. Van Wyk, J. A. Ferreira, IEEE Power Electronics Specialist Conference, 1992, pages 633-638. In addition to using a non-linear saturating inductor, a non-linear capacitor is utilized as a regenerative snubber in the aforesaid topology.
Aside from the technical difficulties referred to above, which arise from increased frequency of operation, prior art resonant pole converters suffer from an important shortcoming in that there are no cost effective methods of regulation of these converters. The obvious solution of a pre-regulator is unacceptable since that would complicate the circuit design, thereby eliminating the most advantageous characteristic of such prior art converters--namely, simplicity of circuit design.
For example, in a recent article entitled "Half Bridge, Self-Oscillating, Multi-Resonant Converter Circuit" by W. A. Peterson and R. Saint-Pierre, 1993, IEEE 0-7803-0982-0/93, pages 77-84, a zero voltage switching circuit is disclosed which is of simple design, but does not offer voltage regulation. The claimed usefulness of this circuit is its ability to provide short circuit protection by limiting the voltage across the resonant capacitor with diode clamps. Under these conditions, a series resonant circuit is formed which utilizes the leakage inductance of the output isolation transformer. The frequency in this circuit is limited to about 100 kHz and the efficiency is approximately 75%.
Accordingly, the inventor has recognized the desirability of a conversion topology which operates at a very high frequency (ie. into the MHz range), which integrates the major device and component parasitics thereby simplifying the circuit design, and which provides a simple means of regulation of the output voltage regardless of line and load variations. The existing topologies do not offer solutions to these problems.